1. Field of the invention:
This invention relates to a semiconductor laser device and more particularly to an index guided semiconductor laser device which attains laser oscillation in a stabilized fundamental transverse mode at an extremely low threshold current level.
2. Description of the prior art:
In conventional semiconductor laser devices, buried-structure laser devices have the lowest threshold current, typical examples of which are buried-heterostructure (BH) laser devices (Japanese Patent Publication Nos. 52-40958, 52-41107, and 52-48066) and double channel planar buried-heterostructure (DC-PBH) laser devices (Japanese Patent Publication Nos. 62-2718 and 62-7719).
FIGS. 8 and 9 show a conventional BH laser device and a conventional DC-PBH laser device, respectively. These buried-structure laser devices oscillate a laser beam according to an index waveguiding operation and have an extremely low threshold current of 20 mA or less. However, if a proper refractive index is not applied to the burying layer 62 which is formed on both sides of the active layer 61, and if a proper width W corresponding to the width of the waveguide is not applied to the mesa, the device will oscillate in a high-order transverse mode. Thus, these buried-structure laser devices are not suitable for use when a laser beam spot is split into two or more. In order for the buried-structure laser devices to oscillate in a fundamental transverse mode, the width W of the mesa must be set in the range of 1 to 2 .mu.m, which causes breakdown of the facets at a relatively low level of optical output power. Moreover, it is difficult to form a mesa with a narrow width, so that mass production of the device cannot be attained.
The index guided semiconductor laser devices also include V-channeled substrate inner stripe (VSIS) laser devices (Appl. Phys. Lett. vol. 40, 1982, p. 372). FIG. 10 shows a conventional VSIS laser device, which is produced as follows: On a p-GaAs substrate 1, a doped n-GaAs current blocking layer 71 (the carrier concentration thereof being 1.times.10.sup.18 cm.sup.-3 or more) is grown. Thereafter, a striped V-channel having the width Wc is formed in the substrate 1 through the current blocking layer 71, resulting in a current path. Then, on the current blocking layer 71 including the V-channel, a p-GaAlAs cladding layer 2, a GaAlAs active layer 3, an n-GaAlAs cladding layer 4, and an n-GaAs cap layer 5 are successively grown, resulting in a double-heterostructure multi-layered crystal for laser oscillation operation. Even when the width Wc of the waveguide is set at a value of as large as 4 to 7 .mu.m, since a laser beam outside of the waveguide within the active layer 3 is absorbed by the current blocking layer 71 and the substrate 1, high-order mode gain is suppressed and a high-order transverse mode does not occur.
However, the threshold current of this VSIS laser device is 40 to 60 mA, which is extremely higher than that of the buried-structure laser devices. Moreover, the VSIS laser device has relatively large astigmatism of 5 to 10 .mu.m. The reason why the threshold current becomes high is that current injected into the device is confined within the inner striped structure formed by the current blocking layer 71, but carrier injected into the active layer 3 diffuses in the transverse direction into the outside of the active layer 3, resulting in carrier unusable for laser oscillation. The unusable carrier results in unnecessary light based on the spontaneous emission and/or generates unnecessary heat, causing a decrease in reliability of the device. On the other hand, the reason why the VSIS laser device has large astigmatism is that light on both sides of the waveguide is absorbed by the current blocking layer 71 and the substrate 1, so that the wavefront of the light is retarded as compared with that of light in the central portion of the waveguide.
In order to solve the problems of both the buried-structure laser devices and the VSIS laser device, as shown in FIG. 11, a buried-VSIS (B-VSIS) laser device in which burying layers 6, 7, and 8 are formed on both sides of the V-channel of a VSIS laser device has been proposed by S. Yamamoto and T. Hijikata, J. Appl. Phys. 61, p. 3108 (1987). The B-VSIS laser device has excellent features of the VSIS laser devices that even when the width of the waveguide is set at a value as large as 4 to 7 .mu.m, stable laser oscillation in a fundamental transverse mode can be attained. Moreover, in the B-VSIS laser device, the diffusion of carrier in the transverse direction within the active layer is prevented by the burying layer, resulting in a low threshold current of 20 mA and less.
However, in the conventional B-VSIS laser device shown in FIG. 11, the outside of the multi-layered crystal positioned over the V-channel is removed by an etching technique from the cap layer 5 to the current blocking layer 71, resulting in a striped mesa for laser oscillation operation. When the multi-layered crystal is grown by liquid phase epitaxy (LPE), a few defects with a diameter of several microns or more, which are referred to as pinholes, often appear on the top surface of the cap layer 5. The number of such defects is as many as there are a few defects within the surface area of a chip (the size thereof being usually 300.times.250 .mu.m). As the outside of the multi-layered crystal disposed on the current blocking layer 71 including the V-channel is removed by an etching technique from the cap layer 5 to the said current blocking layer 71, both the size and depth of pinholes progressively increase, and when the n-GaAs current blocking layer 71 is to be etched, the pinholes pass through the said current blocking layer 71 and reach to the p-GaAs substrate 1. After the mesa-etching process, as shown in FIG. 11, burying layers 6, 7, and 8 are successively grown outside the mesa. The B-VSIS laser device produced in this way is disadvantageous in that the confinement of current cannot be attained at the portion in which pinholes are present, resulting in the occurrence of leakage current. Thus, there is a variation in the threshold current of laser devices formed on a wafer, which causes a decrease in reproducibility, reliability, and mass-producibility of the laser device.
Moreover, the B-VSIS laser device shown in FIG. 11 requires the following three epitaxial growth processes, which causes an increase in the number of manufacturing processes: the growth of the n-GaAs current blocking layer 71; the growth of the double-heterostructure multi-layered crystal comprising the layer elements 2 to 5 for laser oscillation which are formed on the current blocking layer; and the growth of the burying layers 6, 7, and 8. Moreover, the burying layer 7 is grown over the top surface of the cap layer 5, so that current injected into the device cannot flow to the mesa portion.